"I had written a speech because I didn’t trust myself just to stand up there and talk, and this was too important of an issue, I thought, for me to be up there rambling around," she said.

"So the night before I was at 3 a.m. still working on my speech. Yes, I was. And then I heard somebody say that his [Obama’s] staff helped me write the speech, which insulted me, since I was like doing my homework late at night, working on it myself. I didn’t appreciate that," Winfrey said with a laugh.

And she says that even though she’s campaigning for Obama, she’s not against any other candidate.

"I’ve always said this, that my being in support of Barack Obama is not my being against Hillary Clinton or anybody else," Winfrey said. "It’s just that for this moment in time, this is what I know I am supposed to be doing. I feel compelled to do this. So my vote for is not a vote against anybody. It’s just a vote for."

She says that she may go back out on the trail for Obama, but won’t appear in any ads for him.

"That’s not the best use of my time and service," she says.

Denzel Washington, appearing in "The Great Debaters," which Winfrey produced, tells ABC that he’s for Obama.

Meanwhile, new polling shows that Oprah’s appearances are unlikely to nudge Obama higher in the polls and may even hurt him.

According a New York Times/CBS News poll released Monday, only 1 percent of Democrats polled reported that Winfrey’s endorsement would make them more likely to support Obama, while 14 percent said they would be less likely to vote for the candidate because of Winfrey’s support.